Friction Stir Fabrication

ABSTRACT

A low-temperature friction-based coating method termed friction stir fabrication (FSF) is disclosed, in which material is deposited onto a substrate and subsequently stirred into the substrate using friction stir processing to homogenize and refine the microstructure. This solid-state process is capable of depositing coatings, including nanocrystalline aluminum and/or metal matrix composites and the like, onto substrates such as aluminum at relatively low temperatures. A method of making rod stock for use in the FSF process is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. application Ser.No. 12/792,655 filed Jun. 2, 2010, which is a Continuation Applicationof U.S. patent application Ser. No. 11/527,149, filed Sep. 26, 2006,which claims priority to and the benefit of the filing date of U.S.Patent Application No. 60/720,521, filed Sep. 26, 2005, which areincorporated herein by reference in their entirety.

GOVERNMENT CONTRACT

The present invention was supported by the United States Office of NavalResearch under Contract No. N00014-05-1-0099. The United StatesGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to friction stir fabrication, and moreparticularly relates to coating, surface modification and repair ofsubstrates using friction stifling techniques, as well as the productionof friction stir rod stock.

BACKGROUND INFORMATION

Conventional thermal spray coating techniques, such as flame spray,high-velocity oxygen fuel (HVOF), detonation-gun (D-Gun), wire arc andplasma deposition, produce coatings that have considerable porosity,significant oxide content and discrete interfaces between the coatingand substrate. These coating processes operate at relatively hightemperatures and melt/oxidize the material as it is deposited onto thesubstrate. Such conventional techniques are not suitable for processingmany types of substrates and coating materials, such as nanocrystallinematerials due to the grain growth and loss of strength resulting fromthe relatively high processing temperatures.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a low-temperaturefriction-based coating method termed friction stir fabrication (FSF), inwhich material is deposited onto a substrate and subsequently stirredinto the substrate using friction stir processing to homogenize andrefine the microstructure. This solid-state process is capable ofdepositing coatings, including nanocrystalline aluminum and/or metalmatrix composites and the like, onto substrates such as aluminum atrelatively low temperatures. For example, friction stir fabrication maybe used to add new material to the surfaces of 2519 and 5083 Al, thusmodifying the surface compositions to address multiple applicationrequirements. Coatings produced using FSF have superior bond strength,density, and lower oxide content as compared to other coatingtechnologies in use today. The friction stir fabrication process mayalso be used to fill holes in various types of substrates. The presentinvention also provides a method of making friction stirring rod stock.

An aspect of the present invention is to provide a method of forming asurface layer on a substrate. The method comprises depositing a coatingmaterial on the substrate, and friction stirring the deposited coatingmaterial.

Another aspect of the present invention is to provide a method offilling a hole in a substrate. The method comprises placing powder of afill material in the hole, and friction stirring the fill materialpowder in the hole to consolidate the fill material.

A further aspect of the present invention is to provide a method ofmaking consumable friction stirring rod stock. The method comprisesplacing powder of a coating material in a die, friction stirring thecoating material powder in the die to consolidate the coating material,and recovering a rod comprising the consolidated coating material.

These and other aspects of the present invention will be more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d schematically illustrate a friction stir fabricationprocess in accordance with an embodiment of the present invention.

FIGS. 2 a-2 f schematically illustrate a friction stir hole repairmethod in accordance with an embodiment of the present invention.

FIGS. 3 a-3 d schematically illustrate a method of making a consumablefriction stirring rod in accordance with an embodiment of the presentinvention.

FIG. 4 illustrates stress-strain curves for a 5083 Al substrate materialand a 5083 Al nanocrystalline interface for a friction stir coatedsample produced in accordance with an embodiment of the presentinvention.

FIG. 5 is a photomicrograph of a polished friction stir fabricated 5083Al sample corresponding to FIG. 4.

FIG. 6 is a photomicrograph of an etched friction stir fabricated 5083Al sample corresponding to FIG. 4.

FIG. 7 illustrates stress-strain curves for a 5083 Al substrate and afriction stir fabricated 6063 Al—SiC (10 volume percent) coatingdeposited on a 5083 Al substrate.

FIG. 8 is a photomicrograph of a 5083 Al/6063 Al—SiC (10 volume percent)friction stir fabricated sample corresponding to FIG. 7, showing thesubstrate, friction stir fabricated coating, and interfacial regiontherebetween.

FIG. 9 is a photomicrograph of a 2519 Al substrate friction stir coatedwith a 6063 Al—SiC metal matrix composite in accordance with anembodiment of the present invention, including magnified regionsthereof.

FIG. 10 is a series of photomicrographs of an Al—SiC friction stirredcoating in accordance with an embodiment of the present invention.

FIG. 11 is a photomicrograph of a 6063 Al friction stirred coating on a2519 Al substrate produced in accordance with an embodiment of thepresent invention.

FIG. 12 includes an SEM image of the 6063 Al friction stirred coating ofFIG. 11, and corresponding EDS maps for aluminum and copper.

FIG. 13 illustrates photomicrographs of a 5083 Al substrate with a holefilled by a 5083 Al friction stirred material in accordance with anembodiment of the present invention.

FIG. 14 shows Vickers hardness values for various substrate and coatingmaterials.

FIG. 15 is a graph of deposition rate versus translation velocity forfriction stir coating processes of the present invention.

DETAILED DESCRIPTION

A friction stir fabrication process in accordance with an embodiment ofthe present invention includes two steps: coating deposition followed byfriction stir processing. The coating step imparts sufficientinterfacial adhesion such that friction stir processing does notdelaminate the coating from the substrate.

In accordance with an embodiment of the present invention, frictionstirring is used to deposit a coating material on a substrate byfrictional heating and compressive loading of a rod comprising thecoating material against the substrate. The applied load is increased,beyond what would be required to join the rod to the substrate, and theportion of the rod adjacent to the substrate deforms under thecompressive load. The deformed metal is then trapped below a rotatingshoulder and sheared across the substrate surface as the substratetranslates.

FIGS. 1 a-1 d show a step-by-step schematic of the process. FIGS. 1 a-1c illustrate the deposition of material onto the substrate, and FIG. 1 dshows the subsequent friction stir processing used to ensuremetallurgical bonding between the substrate and coating, and tohomogenize and refine the microstructure of the coating.

In the method illustrated in FIG. 1 a, a collar is attached to a rodcomprising the coating material, leaving approximately 3 mm of the rodbeneath the collar. As shown in FIG. 1 b, this 3 mm section is pressedinto the substrate while rotating at approximately 3500 RPM. As shown inFIG. 1 c, the coating material is spread evenly across the surface ofthe substrate with a layer thickness of about 0.4 mm. The collar is thenrepositioned on the filler rod, leaving the bottom 3 mm of the fillerrod beneath the collar, and the process is repeated. As shown in FIG. 1d, once an entire cross-sectional layer is deposited on the surface ofthe substrate, the coated substrate is friction stir processed tohomogenize the new layer and promote interlayer adhesion. Additionallayers may then be applied in a similar manner until the desired coatingthickness is achieved.

An alternative to the friction stirring deposition method describedabove is to deposit the coating material via cold spray, which is arelatively low-temperature thermal spray process in which particles areaccelerated through a supersonic nozzle. However, such cold spraytechniques may be relatively expensive. In addition to its substantialprocessing cost, the cold spray technique is unable to process highaspect ratio particles, such as the nanocrystalline aluminum powderproduced by cryomilling, and the resultant deposited material containsoxide impurities. As such, the friction stirring deposition method maybe preferable to cold spray techniques.

In accordance with an embodiment of the present invention, the coatingmaterial is deposited on the substrate in nanocrystalline form. Afterthe deposited coating has been friction stirred, the nanocrystallinestructure of the coating material may be maintained. As used herein, theterm “nanocrystalline” means a material in which the average crystalgrain size is less than 0.5 micron, typically less than 100 nanometers.Due to the fact that the friction stirring process is carried out at arelatively low temperature below the melting point of the coatingmaterial, little or no crystal grain growth occurs during the frictionstirring process.

In accordance with another embodiment of the present invention, thecoating material comprises a metal matrix composite (MMC). As usedherein, the term “metal matrix composite” means a material having acontinuous metallic phase having another discontinuous phase dispersedtherein. The metal matrix may comprise a pure metal, metal alloy orintermetallic. The discontinuous phase may comprise a ceramic such as acarbide, boride, nitride and/or oxide. Some examples of discontinuousceramic phases include SiC, TiB₂ and Al₂O₃. The discontinuous phase mayalso comprise an intermetallic such as various types of aluminides andthe like. For example, titanium aluminides such as TiAl and nickelaluminides such as Ni₃Al may be provided as the discontinuous phase. Themetal matrix may typically comprise Al, Ni, Mg, Ti, Fe and the like.

To produce Al—SiC metal matrix composite coatings, aluminum tubes may befilled with silicon carbide powder and used as coating rods. The filledtubes may yield an Al—SiC coating, but the volume fraction of thereinforcement may vary locally. However, for precise volume fractioncontrol, homogenous metal matrix composite rods containing theappropriate volume fraction may be used instead of powder filled tubes.

The reinforcement of the metal matrix composite coating may beincorporated into the matrix by traditional blending techniques or grownin-situ from elemental metals using reaction synthesis. Table 1 listsexample MMC systems which can be formed using reaction synthesis. Inreaction synthesis, elemental metals react due to the thermal and/ormechanical energy imparted during processing to form intermetallic orceramic particulates. The rotation of the stirring tool and feedmaterial relative to the substrate may generate frictional heat whichraises the temperature of the elemental constituents to that at whichthe reaction can initiate. As the reactions of elemental metals used forreaction synthesis are exothermic, additional heat is evolved in theformation of the intermetallic particles. An aspect of using FSF to formin-situ MMC coatings is the fact that the shearing of the metal by thestirring tool and rotation of the feed material cracks and disperses theoxide barrier coatings, which exist on all metal exposed to oxygen,providing a high concentration of the metal-to-metal contact requiredfor the reaction to occur. In FSF-based reaction synthesis, the reactingmetal may be provided from the substrate and the feed metal, or all ofthe reacting metals could be provided from the feed material.

TABLE 1 Reaction Synthesis of In-situ MMCs Using FSF Ti + xAl →TiAl + (x− 1)Al (Aluminum matrix with TiAl reinforcement) 3Ni + yAl →Ni₃Al + (y −1)Al (Aluminum matrix with Ni₃Al reinforcement) 2B + zTi → TiB₂ + (z −1)Ti (Titanium matrix with TiB₂ reinforcement) Ti + wNi → NiTi + (w −1)Ni (Nickel matrix with NiTi reinforcement)

In-situ MMCs may exhibit enhanced mechanical properties as compared toMMCs formed ex-situ, i.e., by blending the matrix and reinforcement.In-situ formation of MMCs yields relatively small single crystalreinforcements, which are thermodynamically stable in the matrix.Furthermore, in-situ formation results in clean, unoxidized particles,and thus the interfacial strength between the reinforcement and matrixis higher than that of ex-situ MMCs.

Various types of substrates may be coated using the friction stirfabrication process of the present invention. For example, metalsubstrates comprising Al, Ni, Mg, Ti, Fe and the like may be coated.Furthermore, polymers and ceramics may be provided as the substrate. Forexample, the substrate may comprise a thermoplastic material.

In accordance with an embodiment of the present invention, the coatingmaterial is deposited on the substrate at a temperature below a meltingtemperature of the coating material. For example, deposition may beperformed at a temperature of from 100 to 500° C. or more below themelting point of the coating material. When the coating materialcomprises Al, the material may be deposited on a substrate at atemperature below about 500° C., typically below about 400° C. After thecoating material is deposited, subsequent friction stirring of thematerial is also preferably performed below the melting temperature ofthe coating material. For example, when the coating material comprisesAl, friction stirring temperatures may be maintained below about 500°C., typically below about 400° C. Furthermore, the friction stirringprocess may be performed at a temperature below a melting temperature ofthe substrate.

Another embodiment of the metal deposition method may significantlyreduce the labor and time requirements. In the process, the filler rodis delivered to the substrate surface using a “push” method, where arotating-plunging tool pushes a rod of finite length through therotating spindle. The spindle is rotated independently using anadditional motor while the milling machine rotates the plunging tool. Asthe spindle and plunging tool rotate, the filler rod is pressed into thesubstrate surface with the down force of the plunging tool. This designallows a large volume of raw material to be fed to the substrate surfaceas compared to manual methods. As the rod material is spread onto thesubstrate, the plunging tool continues to feed more filler rod throughthe spindle onto the substrate. For example, up to 75 mm or more offiller rod can be fed through the spindle. With machine designimprovements, the length of the rod stock may be increased.

This “push” method is a feasible solution to the filler rod deliverychallenge, but in the interest of processing speed could be furtherimproved upon. For continuous deposition, a “pull” method, where thespindle rotation pulls the rod into the spindle, may be employed so thatthe rod length can be increased and the rods can be fed continuously. Amethod for pulling the rod into the spindle is to employ a threadedsection on the inner diameter of the spindle throat. During thedeposition process, the spindle rotates at a slightly slower rate thanthe rotating rod stock. Due to the difference in rotational velocities,the threaded portion of the neck pulls the rod through the spindle andforces the metal under the rotating shoulder. The difference inrotational velocity between the rod and the spindle, coupled with thepitch of the internal threads in the spindle, determine the coatingdeposition rate. It may be desired to actively control the temperatureof the rod inside and outside the spindle so that the thermally inducedsoftening of the filler rod is not totally dependent on frictionalheating. Such thermal control provides means to increase depositionrates to meet application requirements.

Another embodiment of the present invention provides a method ofrepairing holes in substrates, and a way to modify the local propertiesof a substrate. A hole repair method is illustrated in FIGS. 2 a-2 f. Asshown in FIG. 2 a, the repair process begins with a substrate having ahole of known diameter. If the hole is not circular in cross-section orhas an unknown or undesired diameter, it may be machined to create ahole equal to the diameter of the stirring tool used in FIG. 2 d. Asshown in FIG. 2 b, if the hole is a through-hole, it may be necessary toapply a backing plate, e.g., composed of either the substrate materialor the filler material. The backing plate serves as a base for thefriction processing to follow, and may be inset into the lower surfaceof the substrate if desired. As shown in FIG. 2 c, a layer of loosepowder is deposited into the hole, and subsequently stirred into thebacking plate or the bottom of the hole, as shown in FIG. 2 d, with astirring tool subsequently equal in diameter to that of the hole. FIG. 2e illustrates the resultant layer of material added to the bottom of thehole. FIG. 2 f illustrates the deposition of more loose powder into thehole, which may be stirred as shown in FIG. 2 d. This process may berepeated until the hole is filled. As the depth of the fill approachesthe top of the substrate, flash material may accumulate around thesurface of the hole. Once the fill depth reaches the substrate surface,the flash material may be cut away leaving a smooth surface.

The hole-repair method may be used to modify the properties of asurface. A series of holes with any given depth may be drilled into asubstrate and then re-filled, using the hole-repair method, with amaterial having the desired local properties, thereby selectivelymodifying the local properties of the substrate. With multiple stirringtools across the work volume, the processing time for an entire workpiece may be reduced, and the ability to selectively vary the localmicrostructure may be readily accomplished.

Because material flexibility is possible using the present process, thedesired alloys and material volume fractions are not always readilyavailable in the rod stock form needed for the raw material. As such, anaspect of the present invention is to provide a friction stir stockfabrication method that uses powder as its raw material. This stockfabrication method provides the ability to produce cylindrical rods froma wide variety of materials and composites in various volume fractions.Further, in contrast to the cold spray coating method, this frictionstir stock fabrication method is able to process high aspect ratioparticles, such as those produced through cryomilling, which allows forthe inexpensive construction of nanocrystalline rods for deposition byfriction stir fabrication.

A variation of the hole filling method may be used for production of rodstock to supply the solid-state friction deposition process describedabove. Because the hole filling method utilizes powder as its rawmaterial, limitless material and volume fraction flexibility exists forproduction of rods and cylinders by this method. For example, thecomposition of the rod stock may be graded along its length, in whichcase coatings made from the rod during the FSF process may havedifferent compositions and properties which vary gradually from one areaof the coating to another, e.g., one area of the FSF coating may haverelatively high hardness while another area may have relatively highcorrosion resistance. To deposit advanced materials such asnanocrystalline aluminum and/or aluminum MMCs using FSF, rod stock ofthese materials with predictable and repeatable volume fractions isdesired. As these advanced materials are not commercially available inrod form, the present low-pressure high-shear powder compaction (LPHSPC)process, as shown in FIGS. 3 a-3 d, may be used to provide rods ofcoating materials for the FSF process.

In one embodiment, LPHSPC may be accomplished by manually depositingapproximately 0.25 g of powder into a cylindrical cavity, asschematically shown in FIG. 3 a, and then manually applying a downwardcompaction force with a spinning cylindrical tool, as shown in FIG. 3 b.As shown in FIGS. 3 c and 3 d, the powder deposition and spinning stepsare repeated. The downward pressure and shear from the spinning toolcompact the powder and adhere it to the previous layer. Fully densesections of, e.g., ⅜ and ½-inch diameter, rods may be fabricated frommicrocrystalline and nanocrystalline aluminum powders using the manualmethod. However, rods of significant length may be fabricated byautomated methods for use as feed stock for FSF systems. Thus,constructing an automated low-pressure high-shear powder compaction unitmay be desirable.

Once the coating has been deposited onto the surface of the substrate,e.g., using the solid-state friction deposition method, it may then befriction stir processed to adhere the coating to the surface of thesubstrate and refine the coating microstructure. The goal of thefriction stir process is to produce a homogenous coating with a bondstrength approaching the ultimate tensile strength of the base alloy.The quality of the friction stirred regions of the substrates may beoptimized, including eliminating any channel present along the length ofthe friction stir path. Elimination of the channel may be achieved byusing a friction stir tool with a threaded pin. By modifying thestirring tool geometry, coated substrates may be produced withoutchannels through the use of a threaded-tapered stirring tool.

The following examples are intended to illustrate various aspects of theinvention, and are not intended to limit the scope of the invention. Inthe following examples, different deposition geometries are used to testthe bond strength between 5083 Al and a ½ inch deposit ofnanocrystalline Al (7 w % Mg, cryomilled 4 hrs); and test the bondstrength between 5083 Al and a ½ inch deposit of 6063 Al—SiC (10 v %).Small tensile specimens were cut such that the 5083 Al substrate and thecoating (nanocrystalline Al or Al—SiC) each composed half of thespecimen and the interface plane between the coating and substrate wasin the middle of the gauge length, normal to the loading direction.

Friction stir fabrication was used to coat 2519 and 5083 Al substratesas follows: 2519 and 5083 Al plates with Al—SiC surface layers—theAl—SiC coating was comprised of 6063 Al and approximately 10 v % SiCpowder (1 mm average particle size); A 2519 Al plate with a copper-freesurface to enhance the corrosion resistance—the copper-free coating wasmade from 6063 Al; A 5083 Al plate with a nanocrystalline aluminumdeposit to enhance the impact resistance—the nanocrystalline aluminumalloy contained 7 w % Mg, and was cryomilled for 4 hours; A half-inch,curved Al—SiC rib on a 5083 Al plate—the rib was composed of 6063 Al andapproximately 10 v % SiC powder (1 mm average particle size); and repairof a one-inch diameter hole in a 5083 Al plate without adverselyaffecting the plate microstructure—the material used for the repairprocess was either commercially pure Al or nanocrystalline Al (due tomachine limitations, the diameter of the hole was reduced to ahalf-inch).

EXAMPLE 1

5083 Al plate was coated with nanocrystalline aluminum deposit. Becausethe nanocrystalline Al was a limited supply and in powder form, thecoating deposit was made using the “hole-filling” method and theinterface at the bottom of the hole was the interface of interest. FIG.4 shows representative stress-strain curves for bulk 5083 Al, and the5083 Al substrate with nanocrystalline Al coating. The micrographs ofFIGS. 5 and 6 show the interfacial region between the substrate and thedeposit as polished and etched, respectively. The microstructure in thenanocrystalline region is very fine while the 5083 is characterized bylarge precipitates and large high aspect ratio grains.

Consolidated deposits of nanocrystalline Al powder are preferablyhomogenous and fully dense. All of the 5083 Al-nanocrystalline Altensile specimens tested failed at or near the interface atapproximately 75-95% of the bulk 5083 ultimate tensile strength,indicating that metallurgical bonding occurred between the base metaland the deposit.

The range of bond strengths measured was 227-285 MPa, at least 2.5 timeslarger than any of the bond strengths reported for thermal spraycoatings (Table 2). The hardness of the 5083 Al and nanocrystalline Alwere measured to be 78.1±2.5 HV and 108.5±7.5 HV respectively (FIG. 14summarizes the FSF coating hardness values), indicating that afterconsolidation the nanocrystalline Al retains strength superior to 5083Al.

EXAMPLE 2

An aluminum substrate was coated with an Al—SiC metal matrix composite.SiC-powder-filled 6063 Al tubes were used as the deposition material forsamples with an Al—SiC MMC coating. The matrix for the MMC coating maybe commercially pure (CP) Al, however, CP Al tubes of the desireddiameter may not be readily available. Therefore, 6063 T5 Al tubes maybe substituted for CP Al tubes for this demonstration. 6063 Al wasselected because it contains silicon, which limits the dissolution ofsilicon from the silicon carbide reinforcement. Such dissolution wouldlead to the formation of Al₄C₃, a detrimental brittle phase. The averageparticle size (APS) of the SiC powder used was 1 mm and the volumefraction of SiC in the composites was approximately 10 vol. %.

EXAMPLE 3

A 5083 Al plate was coated with an Al—SiC metal matrix composite. Totest the bond strength between 5083 Al and 6063 Al—SiC (10 v %), a½-inch thick MMC coating was deposited on a 5083 Al substrate using FSFwith SiC filled 6063 Al T5 tubes as the feed rod. FIG. 7 shows astress-strain curve for the 5083 Al substrate and the interface betweenthe substrate and the Al—SiC metal matrix composite coating. Across-section of the polished MMC coating and substrate are shown on theright side of FIG. 8. Significant improvements in both the coating andinterfacial microstructure have been made. The improvements primarilyresult from the use of a threaded-tapered stirring tool forpost-deposition friction stir processing. A friction stir processingpass was made (the stirring tool translated normal to the cross-sectionshown in the micrograph) after each incremental increase in the coatingthickness of approximately ⅛-inch. As is evident from the micrograph,the friction stir processed (FSP) zone has a relatively homogeneousmicrostructure while the areas to the left and right of the FSP zoneexhibit a layered heterogeneous microstructure. In the FSP zone, theinterface between the substrate and the MMC is diffuse, and SiCreinforcement is present approximately 2 mm below the original substratesurface. The inset micrograph in the middle of the figure shows the areaof maximum SiC penetration.

The continuity of the aluminum matrix throughout the interfacial regionand into the substrate indicates that metallurgical bonding occurredbetween the MMC and substrate. Tensile specimens were cut from thecoating/substrate on the vertical mid-line of the FSP zone with theinterface in the center of the gauge length, normal to the loadingdirection. FIG. 7 shows a representative stress-strain curve for thecoating/substrate tensile specimens and for bulk 5083 Al; the ultimatetensile strength (UTS) of 6063 T1 Al is also indicated on the graph.Failure of the coating/substrate tensile specimen occurred in the gagelength at 157 MPa on the MMC side of the interface; significant neckingwas observed in the MMC. All coating/substrate tensile specimens failedin the MMC half of the sample due to the low strength of the 6063 Almatrix alloy. 6063 Al has an ultimate tensile strength of 150 MPa in theT1 condition (cooled from fabrication temperature and naturally aged).

The bond strength of the coating/substrate interface nearly doubles thatof the best available competing thermal spray process.

EXAMPLE 4

A 1.5 mm thick Al—SiC MMC coating was deposited on a 2519 Al substrateusing the FSF process in a manner similar to that of Example 3. Themicrograph on the right side of FIG. 9 shows the coating/substrateinterfacial region, which occurs below the original substrate surface.As observed in the MMC coated 5083 sample, the metal matrix iscontinuous through the thickness of the interfacial region and into thesubstrate indicating that metallurgical bonding has occurred between thecoating and substrate. The micrograph on the top left in FIG. 9 showsthe MMC coating as well as friction stir processed and unstirred 2519after etching. It is evident from the micrograph that the microstructurein the FSP zone has been refined and the grain size significantlyreduced. The macro-Vickers hardness of the MMC coating in the frictionstir processed zone and the un-stirred zone are 56±4 HV and 59±4 HV,respectively. The hardness of FSF 6063 Al is 47±3 HV (FIG. 14). Thus,addition of approximately 10 vol % SiC results in a 20% increase in thecoating hardness.

EXAMPLE 5

A curved Al—SiC rib (2.5 mm tall, 13 mm wide, 90 mm long) was built on a5083 Al plate using the solid-state metal deposition method. Visually,it is clear that the silicon carbide particulates have been incorporatedinto the 6063 Al matrix and the rib material has been adhered to thesubstrate. FIG. 10 shows four micrographs of the Al—SiC rib material atdifferent magnifications. Friction stir processing of the Al—SiC ribshown in these micrographs was done using a stirring tool with anunthreaded cylindrical pin. The use of this stirring tool resulted insome variation in the local SiC volume fraction (bottom two micrographs)and a channel at the bottom of the FSP zone. Subsequent processing ofthe same MMC coating and 5083 Al substrate with improved tool geometryproduced homogeneous coatings without a channel, as described in theprevious sections.

The lowest magnification image in FIG. 10 (upper left) shows a corner ofthe rib on the retreating side of the friction stir pass; it is apparentthat some inhomogeneity exists in the local SiC volume fraction. Theupper right micrograph shows the interfacial region at the edge of theFSP zone. No discontinuity between the matrix and substrate is observedand a banded dispersion of SiC exists due to repeated FSP of the rib.

This experiment demonstrates that the FSF process has the ability todeposit discontinuously reinforced metal matrix composites in varyingand complex shapes. The process is not limited by shape or height, andproduces structures with no discrete interface between the depositedstructure and the substrate.

EXAMPLE 6

A 1.25 mm thick surface layer of copper-free 6063 Al was added to a 2519Al plate (approximately 4×4 inches) using friction stir fabrication.Commercially pure (CP) Al may be specified for coating the 2519 surface,however, CP Al rods in the desired diameter may not be readilyavailable. Therefore, 6063 Al may be substituted for CP Al for thisdemonstration because 6063 Al contains no copper and has relatively goodcorrosion resistance.

The resulting structure is as desired, a coherent coating thatcompletely shields the more corrosive 2519 Al from the surface. FIG. 11shows micrographs of the coating and substrate in the as polished stateand etched conditions. The microstructure in the FSP zone has beenrefined and the grain size significantly reduced. The interface betweenthe substrate and coating shows no visible porosity and exhibitsbanding, alternating layers of coating and substrate material. Thehardness of FSF 6063 Al coating on the 2519 Al substrate was determinedto be 47±3 HV (FIG. 16).

This demonstrates the feasibility of adding corrosion-resistant materialto the surface of a substrate using the present friction stirfabrication process. FIG. 12 shows a scanning electron microscope (SEM)micrograph (left) and elemental maps of Al (middle) and Cu (right)obtained by energy dispersive spectroscopy (EDS). The EDS maps show thatthe 6063 Al coating provides a copper-free layer on top of the 2519substrate. Further, there is no limit on the thickness of the materialthat can be added to the substrate due to the additive nature of the FSFprocess.

EXAMPLE 7

A hole was repaired in a 5083 Al plate. Multiple holes in 5083 plateswere repaired/filled with commercially pure aluminum or nanocrystallinealuminum using the hole-repair method similar to that shown in FIG. 2.The diameter of the hole was one half-inch for this demonstration. FIG.13 show micrographs of a portion of the bottom and outer-diameter of ahole repaired with nanocrystalline aluminum in the polished and etchedstates. No porosity is observed between the stirred layers or at theinterface of the hole. The discontinuous porosity that was observed andreported in previous progress reports has been eliminated throughprocess improvements. A large heat-affected zone exists surrounding thehole, showing that significant heat and shearing forces were present asa result of the repeated stirring action.

Friction stir fabrication is a solid-state process capable of depositingcoatings, including nanocrystalline aluminum and/or metal matrixcomposites, onto aluminum substrates at relatively low temperatures.Coatings produced using FSF have superior bond strength, density, andoxidation characteristics as compared to other coating technologies inuse today. Mature thermal spray technologies, such as flame spray,high-velocity oxygen fuel (HVOF), detonation-gun (D-Gun), wire arc andplasma deposition, produce coatings that have considerable porosity,significant oxide content and a discrete interface between the coatingand substrate. These coating processes operate at relatively hightemperatures and melt/oxidize the material as it is deposited onto thesubstrate. Therefore, these technologies are not suitable for processingnanocrystalline materials due to the resulting grain growth and loss ofstrength.

The major process and coating characteristics for common thermal sprayprocesses are listed in Table 2 in comparison with friction stirfabrication processes in accordance with embodiments of the presentinvention. In addition to high operating temperatures, anothersignificant drawback to conventional thermal spray coating is relativelylow bond strength. The bond strengths of thermal spray processes arerelatively low because there is limited metallurgical bonding to thesubstrate due to the lack of mechanical and/or thermal energy impartedto the substrate during coating. Thermal spray coating could be comparedto soldering or brazing; the substrate or base metal is notmetallurgically bonded to the coating via a long-range diffuseinterface.

TABLE 2 Capabilities of Existing Coating Processes and FSF FSF 6063 FSFnano Al—SiC Plasma x-tal on MMC on Flame Spray HVOF D-Gun Wire Arc ColdSpray Spraying 5083 Al 5083 Al Heat Source Oxyacetylene Fuel gasesOxygen/ Electric arc Resistance Plasma arc Friction Friction Acetyleneheater detonation Typical 3000 3000 4500 >3800 20-700 16000 ~350 ~350 Processing Temperature (° C.) Relative 85-90 >95 >95 80-95 97-99 90-99 >99  >99 Density (%) Bond Strength,  7-18 68 82 10-40 70 (est.) 68227-285*  >150* (MPa) Oxides High Moderate Small Moderate to PriorModerate None None to high particle to coarse observed observeddispersed boundaries *Experimental results show that the bond strengthis approximately equal to the ultimate tensile strength of the weakestcomponent, substrate or coating.

The FSF process may be used to meet coating needs, e.g., coatingnanocrystalline Al and Al MMCs onto vehicle armor for enhanced ballisticimpact resistance. Of interest is the bond strength between the FSFcoating and the base armor because the through-thickness mechanicalproperties of a layered system often never approach those of theindividual components due to relatively low bond strength.

The materials produced in accordance with the present invention may beused for various applications such as ballistic impact resistant armor.For example, for a particular vehicle to achieve the survivability andweight reduction objectives, the ballistic impact resistance of thearmor of the vehicle should be enhanced through the use of high-strengthadvanced engineering materials such as nanocrystalline aluminum and/oraluminum metal matrix composites (MMCs). The strengths (two to threetimes that of the bulk microcrystalline alloy) and reasonableductilities (approximately 4%) of these advanced aluminum-basedmaterials make them ideal candidates for ballistic coatings on 2519 and5083 Al armor plate.

In addition to providing enhanced ballistic impact resistance, coatingthe base armor plate also has the potential to mitigate corrosionproblems present in copper rich alloys such as 2519 Al. Furthermore, theuse of nanocrystalline aluminum for bosses which serve as attachmentpoints for armor panels, electronic components, seats, and otherequipment on the EFV would realize additional weight savings andstrength improvements. For these advanced materials to be deployed, acost effective method for depositing thick coatings with minimaldeleterious effects on the microstructure of the substrate and coatingmaterial must be developed. Current thermal spray technologies are notsuited for depositing these advanced Al-based materials, primarily dueto the high processing temperatures, which lead to significant graingrowth and loss of strength.

The FSF coating process of the present invention imparts significantshear stresses on the coating/substrate interface, resulting in bondstrengths significantly higher than those observed in thermal spraycoating processes. Additionally, because FSF is a solid-state process,it is more suited to the processing of grain growth-prone materials suchas nanocrystalline aluminum.

Factors that influence the deposition rate are translation speed,shoulder diameter, layer thickness, and delays resulting from manualprocesses. The angular velocity of the spindle is an important variablefrom the perspective of frictional heating and deposition quality, butdoes not directly factor into the deposition rate unless poor depositionquality leads to necessary rework. Once the acceptable angular velocityrange for the spindle is established for a given coating material, thisvariable will no longer have an impact on the deposition rate but couldbe used to manipulate the frictional heat input and thus the structureand properties of the coating. The deposition efficiency of the FSFprocess is nearly 100%. Material waste (scrap) in the FSF process occursonly when machining flash at the edge of the FSP region. This waste canbe minimized or eliminated in a number of ways, including process andproduct design.

A spindle capable of continuous deposition will eliminate manualintervention and setup delays, and allow material to be continuously fedthrough the spindle to the substrate surface. For continuous deposition,the material deposition rate will be equal to the product of thetranslation speed, shoulder diameter, and layer thickness. FIG. 15illustrates the relationship of these process variables to thedeposition rate. Given a layer thickness of 0.035 inches (0.9 mm), tomeet the goal of 30-40 cubic inches of deposition per hour (1.3-1.6kg/hour for Al), the translation speed must be increased to 10-16 inchesper minute (250-410 mm/min) for a shoulder diameter in the range of0.75-1.25 inches (19-32 mm). Long-term, the deposition rate should beimproved to equal or exceed that of HVOF and other mature thermal spraytechnologies.

Friction stir fabrication is an effective and potentially efficientmethod of producing a variety of aluminum-based coatings. Using a manualdeposition method, the FSF process was able to produce coatings, fromadvanced materials in the solid-state, with at least twice the bondstrength of the most competitive coating technology. In addition, a widevariety of aluminum feed stock for FSF can be fabricated using thepowder compaction process, allowing for wide-ranging materialflexibility in FSF coatings. It may be desirable to provide an automatedcoating unit that can perform reproducibly over a wide range of processparameters and is capable of in-situ process monitoring. Consistentperformance and the ability to monitor spindle speed, torque, anddeposition temperature will afford the ability to detail the linkbetween the FSF process and the coating structure and properties. Oncethe process-structure-property relationship map has been established andthe key process elements and parameters identified, process developmentcan then focus on designing automated FSF equipment with enhanceddeposition rates.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention.

1. A method of forming a surface layer on a substrate comprising:providing a stirring tool comprising a shoulder and a throat; providinga coating material disposed within the throat of the stifling tool;orienting the stirring tool relative to a substrate such that theshoulder opposes an outer surface of the substrate; rotating thestirring tool and coating material together at a selected RPM; imposingrelative lateral movement between the stirring tool and the substrate;depositing the coating material on the substrate in a manner to form acoating on the substrate in a volume between the shoulder and the outersurface of the substrate; and forming and shearing a surface of thecoating by way of frictional heating and compressive loading of thecoating material against the substrate.
 2. The method of claim 1,wherein the depositing involves applying a force to the coating materialto force the coating material out of the tool.
 3. The method of claim 1,wherein the depositing comprises depositing the coating material on thesubstrate by plastically deforming and combining both a portion of thecoating material and a portion of the substrate to form a coating on thesubstrate in a volume between the shoulder and the outer surface of thesubstrate.
 4. The method of claim 1, wherein the spinning of thestirring tool and the coating material is performed at a desired angularvelocity.
 5. The method of claim 4, wherein the stirring tool and thecoating material are spinning at the same desired angular velocity. 6.The method of claim 1, wherein the throat and coating material each havea surface for engaging one another to provide rotational velocity to thecoating material upon rotation of the stifling tool.
 7. A method offorming a surface layer on a substrate comprising: providing a stirringtool comprising a shoulder, a throat, and no pin; providing a coatingmaterial disposed within the throat of the stifling tool; orienting thestirring tool relative to a substrate such that the shoulder opposes anouter surface of the substrate; rotating the stirring tool and coatingmaterial; imposing relative lateral movement between the stirring tooland the substrate; depositing the coating material on the substrate in avolume between the shoulder and the outer surface of the substrate; andforming and shearing a surface of the coating by way of frictionalheating and compressive loading of the coating material against thesubstrate.
 8. The method of claim 7, wherein the depositing involvesapplying a force to the coating material to force the coating materialout of the tool.
 9. The method of claim 7, wherein the depositingcomprises depositing the coating material on the substrate byplastically deforming and combining both a portion of the coatingmaterial and a portion of the substrate to form a coating on thesubstrate in a volume between the shoulder and the outer surface of thesubstrate.
 10. The method of claim 7, wherein the spinning of thestirring tool and the coating material is performed at a desired angularvelocity.
 11. The method of claim 10, wherein the stirring tool and thecoating material are spinning at the same desired angular velocity. 12.The method of claim 7, wherein the throat and coating material each havea surface for engaging one another to provide rotational velocity to thecoating material upon rotation of the stifling tool.
 13. A method offorming a surface layer on a substrate comprising: providing a stirringtool comprising a throat and a shoulder; providing a coating materialdisposed within the throat of the stifling tool; orienting the stirringtool relative to a substrate such that the shoulder opposes an outersurface of the substrate; rotating the stirring tool and coatingmaterial on an axis of rotation, wherein the stifling tool has no pinconfigured to rotate on the axis of rotation; imposing relative lateralmovement between the stirring tool and the substrate; depositing thecoating material on the substrate in a volume between the shoulder andthe outer surface of the substrate; and forming and shearing a surfaceof the coating by way of frictional heating and compressive loading ofthe coating material against the substrate.
 14. The method of claim 13,wherein the depositing involves applying a force to the coating materialto force the coating material out of the tool.
 15. The method of claim13, wherein the depositing comprises depositing the coating material onthe substrate by plastically deforming and combining both a portion ofthe coating material and a portion of the substrate to form a coating onthe substrate in a volume between the shoulder and the outer surface ofthe substrate.
 16. The method of claim 13, wherein the throat andcoating material each have a surface for engaging one another to providerotational velocity to the coating material upon rotation of thestifling tool.
 17. The method of claim 13, wherein the spinning of thestirring tool and the coating material is performed at a desired angularvelocity.
 18. The method of claim 17, wherein the stirring tool and thecoating material are spinning at the same desired angular velocity. 19.The method of claim 13, wherein the shoulder has a flat surfacegeometry.
 20. The method of claim 13, wherein the shoulder has ageometry with structure for enhancing mechanical stirring of the coatingmaterial during the depositing.